1. Field of the Invention
This invention relates to an acrylic elastomer and its vulcanizable composition, and more particularly to an epoxy group-containing acrylic elastomer capable of producing a vulcanization product having a considerably high compression set and heat resistance even if the secondary vulcanization is omitted, and its vulcanizable composition.
2. Description of the Prior Art
Vulcanizable systems of acrylic elastomers having epoxy groups as cross-linking sites generally contain polyamines, polyamine salts, dithiocarbamates, organic carboxylic acid ammonium salts, imidazoles, etc. as cross-linking agents. However, these vulcanizable systems require a prolonged secondary vulcanization time, because of lower vulcanization rates, as compared with vulcanizable systems of acrylic elastomers containing halogens or active halogens as cross-linking sites, for example, vulcanizable systems containing sulfur or sulfur donor compound-alkali metal salt of fatty acid, trithiocyanuric acid-alkali metal salt of fatty acid, dithiocarbamates, etc. as cross-linking agents, and furthermore they have such disadvantages that the secondary vulcanization products have poor compression set and heat resistance.
However, the acrylic elastomers having epoxy groups as cross-linking sites have such an advantage as a lower metallic corrosion and thus improvements of the vulcanization rate and the compression set have been investigated for the effective utilization of acrylic elastomers having epoxy groups as cross-linking sites.
That is, the following cross-linking agents have been proposed for the vulcanizable system:
Japanese Patent Application Kokai (Laid-open) No. 61-26620 proposes compounds containing at least two carboxyl groups in the molecule-quaternary ammonium salt or phosphonium salt;
Japanese Patent Application Kokai (Laid-open) No. 61-235424 proposes polycarboxylic acid anhydrides-quaternary ammonium salt or phosphonium salt;
Japanese Patent Application Kokai (Laid-open) No. 62-236849 proposes polycarboxylic acids (anhydrides)-quaternary ammonium salt or phosphonium salt-urea;
Japanese Patent Applications Kokai (Laid-open) Nos. 63-57628 and 63-57629 propose organic compounds having at least two --CXNHCY-- bonds (wherein X and Y: O or S) in the molecule-quaternary ammonium salt or phosphonium salt (-compound having a urea bond or thiourea bond); and
Japanese Patent Application No. 63-218752 proposes organic compounds having at least two --CXNHCY-- bonds (wherein X and Y:O or S) in the molecule-guanidine-based compounds.
As to acrylic elastomers having epoxy groups as the cross-linking sites in combination with other cross-linking sites or their vulcanizable systems, the following acrylic elastomers have been proposed:
Japanese Patent Application Kokai (Laid-open) No. 61-26621 proposes acrylic elastomers having epoxy groups and carboxyl groups in combination with the vulcanizable system based on quaternary ammonium salt or phosphonium salt-urea or amide; and
Japanese Patent Application Kokai (Laid-open) No. 61-73708 proposes acrylic elastomers comprising copolymers of acrylic acid ester-dienic monomer-epoxy group-containing vinyl monomer (-monovinylic or monovinylidenic monomer).
However, these acrylic elastomers or their vulcanizable systems also require a secondary vulcanization in order to obtain vulcanization products having a high compression set. Furthermore, changes in the physical properties in the normal state before and after the secondary vulcanization are large and no higher heat resistance is obtained.